The use of wireless telecommunications has undergone substantial growth in the past decade and is projected to continue expanding as service improves and new products and features are offered. In recent years, certain standards adopted by the communication industry provide the capability for increased user capacity and improved privacy by adding digital speech synthesis capability to mobile stations and basestations. Until all wireless telecommunications networks implement the capability to provide digital transmission, however, mobile stations must have circuitry capable of receiving and processing both conventional analog, as well as digital, signals.
Conventional analog wireless communications are based on frequency modulation ("FM") of the radio channel. As will be explained in more detail hereinafter, signal amplifier circuits within analog radio receivers are typically operated in a "limiting," or "saturated," condition. In contrast, several of the industry standards for digital receivers require that the signal amplifiers be operated in a linear (i.e. "non-saturated") mode under most circumstances. Thus, conventional mobile stations capable of operating in both digital and analog modes use a dual-mode intermediate-frequency ("IF") receiver having separate and independent amplifier chains for each mode; i.e. an analog mode IF amplifier chain and a digital mode IF amplifier chain.
Many wireless telecommunications standards mandate that mobile stations provide a received signal strength indication ("RSSI"). The RSSI information is used by the wireless telecommunications system to set transmitter power levels, assign frequency channels, and assign mobile units to base stations. Conventional dual-mode receivers derive the RSSI signal from the analog mode IF amplifier chain when operating in analog mode. When operating in digital mode, the RSSI signal is typically derived by quantization of the baseband waveform demodulated from the output of an automatic-gain-control ("AGC") amplifier that is used to control the gain of the digital-mode IF amplifier chain. Although the RSSI signal derived from the analog mode IF amplifier chain could be used in digital mode, it would be necessary to provide power to both the digital and analog mode IF amplifier chains during digital mode operation, which would approximately double the amount of power required to operate the IF amplifier section of the receiver. On the other hand, deriving the RSSI signal by quantization of the baseband waveform demodulated from the output of the digital-mode AGC amplifier requires additional baseband processing circuitry.
From the foregoing, it is easy to recognize that the coexistence of separate and independent analog and digital mode IF amplifier chains has several disadvantages. First, according to conventional designs, providing both analog and digital modes of operation requires the use of separate and independent analog and digital mode IF amplifier chains. Thus, much more circuitry is required, which may increase the size and weight of the mobile station. Furthermore, if it is desired that the circuitry be implemented in an integrated circuit, the cost of the mobile station may be increased. This is because, in the semiconductor industry, there is a cost associated with the size (area) of the silicon die, or "chip," on which an integrated circuit is fabricated. A characteristic of the defect probability associated with fabricating semiconductor wafers is that the cost increases as a substantially exponential function of the die size. Therefore, there is an incentive to minimize the die area, which can only be accomplished by reducing the amount of circuitry. Secondly, and of particular importance, mobile stations are typically powered by batteries, and thus it is desirable to minimize power consumption to thereby extend the operating time of the mobile station. Consequently, there is a further incentive to improve the power efficiency of the mobile station by eliminating circuitry.
Therefore, what is needed in the art is a way of integrating the functions of the conventional dual IF-amplifier chains into a single "dual-mode" amplifier chain capable of operating in both digital and analog modes.